Rapid, efficient and accurate inspection of large industrial and machine parts is critical to determining defect existence and severity. Based on the inspection results, the user can determine whether the part can be effectively repaired or requires replacement. One such part of interest is the exhaust transition duct that carries hot exhaust gases from the combustion zone of a combustion turbine to external exhaust processing components. The gas combustion process imparts rotational energy to a shaft of an electrical generator, producing electricity for a power delivery system or for an industrial site. Certain gas turbines include as many as 16 exhaust transition ducts, each duct including as many as 20 individual and differently shaped panels welded together to form the duct. The panel material typically comprises a nickel-based super alloy.
During gas turbine operation, hot hydrocarbon-laden exhaust gasses, with temperatures up to 2000° F., pass through the exhaust duct. These severe environmental conditions can cause the ducts to develop various defect types and sizes, both in the panel surfaces and in the welds between the individual panels. The gases are corrosive and experience wide temperature excursions, causing the ducts to flex, erode and crack. A thermal barrier layer overlies the duct exterior surface to retain heat within the duct structure and avoid heating (and thus damaging) proximate components of the gas turbine and electrical generator, such as wires and electrical devices. Conventionally, the thermal barrier comprises two materials, a metallic substrate or bond coat, and a ceramic topcoat. The thermal barrier is applied using a thermal spray process with stringent process controls. Thus application of the thermal barrier layer can be a time-consuming and relatively expensive process.
To ensure the ducts retain the required structural integrity, they are subjected to frequent inspections designed to identify defects. The defects are classified according to defect size, the number of defects within a predetermined distance of each other, and the location of a defect relative to a particular feature of the duct (e.g., features such as the inlet face or the exhaust mouth). Once the defects have been identified and classified, predetermined threshold defect parameters are consulted to determine whether the defects impair the duct structural integrity, and whether the duct should be repaired or replaced.
Prior art surface inspection processes useful for detecting defects in metal objects, such as the exhaust transition ducts, include visual inspection and dye penetrant inspection. When employed to inspect the exhaust gas transition ducts, both of these inspection techniques require removal of the thermal barrier layer prior to performing the inspection. If the detected defects are of such a character that they can be repaired, then following the repair the thermal barrier coating must be reapplied.
The nature of a visual inspection process is self-evident. After the thermal barrier layer is removed, a repair technician visually inspects the individual duct panels and the joining welds to identify cracks, discontinuities or other defects. If not identified and not timely repaired, such cracks or defects can grow; eventually causing a breach in the panel through which the hot exhaust gases can escape. Only surface defects are detectable according to this technique. The visual inspection process is subjective and dependent on the skill of the inspector.
The known non-destructive dye penetrant inspection technique requires a clean material surface, free of surface films and oxides. Thus the barrier layer of the exhaust transition duct must be removed. The area is then flooded with a penetrant composition (commonly comprising a light hydrocarbon oil or an emulsifiable oil) that contains either a visible penetrant (typically a red dye) or a fluorescent dye. The penetrant is permitted to stand on the surface of the material for a sufficient time to seep into surface discontinuities or cracks that extend to the surface. The excess penetrant is removed, and after drying, a developer is applied to the surface. The developer may take the form of finely ground solid particles or a dispersion of solid particles in a liquid or an aerosol. The deposition of these particles immobilizes the penetrant and renders it contrastingly visible. Inspection of the piece is then conducted under ordinary white light, in the case of a visible penetrant, or under ultraviolet radiation, in the case of a fluorescent dye penetrant. Like the visual inspection process, only the surface defects are detectable, and the efficacy of the process depends on the inspector's skill level.
It is noted that both the visual and dye penetrant inspection processes reveal only surface defects. Neither the inwardly facing surfaces nor the interior regions of the transition duct panels is inspectable using these processes. In an effort to overcome certain of these limitations, the duct can be subjected to a heat-treating process prior to the visual or dye penetrant inspection. Heating the duct may cause internal defects to migrate to the surface, where they can be identified according the visual or dye penetrant tests.
X-ray radiography is another known non-destructive inspection process for producing an image representing the density of an object, such as a panel of the exhaust transition ducts discussed above. Low-density regions, such as voids, are visible in the radiograph due to their contrast with high-density regions. High energy radiation, typically x-rays or gamma-rays, is transmitted through the object, attenuated as a function of the object density along the energy path between the source and the detector, and converted into light of a corresponding intensity as the transmitted rays impinge on a detector screen. The screen is conventionally constructed with phosphor particles that absorb the transmitted x-rays and convert them into visible light or ultraviolet radiation. A photographic film, conventionally comprising a silver halide emulsion layer, is responsive to the visible light or the ultraviolet radiation for changing the characteristics of the emulsion layer. The film is developed to reveal an image conforming to the transmission (or conversely, attenuation) of the incident energy passing through the object. Use of a film system is preferred as film exhibits a higher sensitivity to the secondary light or ultraviolet radiation emitted by the detector screen than to the direct impact of the transmitted x-rays.
There are known limitations to the film x-ray radiography technique. The resulting image is static and based solely on the source x-ray characteristics. If it is desired to create a different image to discern different details of the object, the image must be re-shot using source x-rays of a different energy level and/or incident from a different direction. Because more dense regions of the object induce greater attenuation of the x-ray beam, object density can be determined from the resulting film product. However, an x-ray does not indicate density as a function of depth through the object, that is, along the incident x-ray beam. The x-ray film thus provides only a two-dimensional representation of a three-dimensional object. If an x-ray reveals a denser region in a corner of the object, for example, it is not possible to determine whether that denser region is on the incident surface (i.e., with respect to the impinging x-ray) of the object, on the surface where the x-ray exits the object or in an interior region between these two surfaces.
Additionally, since the various duct surfaces and the thermal barrier layer present several different thicknesses to an impinging x-ray, it is necessary to use x-rays of different source energies dependent upon the thickness of the imaged region. The welds, for example, present a thicker material than the panels and thus require a higher energy incident x-ray to produce a usable image. However, it is known that higher energy x-rays reduce the contrast between regions of unequal density, thus reducing the ability to distinguish regions of different densities.
Further, to image a complex three-dimensional structure such as the transition exhaust duct, it is necessary to reposition both the receiving film and the x-ray source prior to each image. All of these complexities associated with x-ray radiography contribute to longer inspection times and therefore increased inspection costs.